Regenerate tournament matches

Recovery lever for an IN_PROGRESS tournament whose match generation partially failed - typically after POST /tournaments/{identifier}/start, whose background chain gives each node one attempt before moving on. This starts a FRESH generation pass with an empty skip list, so every currently-eligible node is re-attempted. It is safe to call repeatedly: generation is exactly-once per node, so a node that already has a match is never regenerated and existing matches are never destroyed. A tournament that is not IN_PROGRESS answers 400. The work is ASYNCHRONOUS - queued is an upper bound on what will be re-attempted, not a completion count; poll GET /tournaments/{identifier}/bracket for the real state. Requires a SEPARATE regenerate capability on the key (it is not covered by lifecycle), and the key owner must be a tournament ADMIN - a tournamentStaff MODERATOR is NOT sufficient and receives 403.